how do i know when to drop a vowel or add a fusion consonant, when 2 vowels come together ?

i understand major vowel harmony, changing the vowel in the suffix to either soft or hard depending on the last vowel of the word before.

and i understand minor vowel harmony, changing suffix´s begining with -i or -ý to -i,-ý,-u,-ü depending on the last vowel of the word before it, but how do i know which vowel harmony to use, because i dont understand how to use them both together.  or can i use them both together? 

can i just learn this rule? if i add a suffix starting with a vowel to a word, i change the vowels of the suffix to:

-ý if the vowel in the word before is -a or -ý

-u if the vowel in the word before is -o or -u 

-i if the vowel in the word before is -e or -i 

-ü if the vowel in the word before is -ö or -ü

any help or advise would be appreciated . 

A Turkish word can have either all soft or all hard vowels (except for words adopted from foreign languages, like tarih - it has both hard vowel "a" and soft vowel "i", so you know it´s not an originally Turkish word, because it doesn´t follow Major Vowel Harmony). For example melek  is a Turkish word - it has only soft vowel "e" - so you know that the suffix you use needs to have soft vowels - either -e or -i.

What you do with the vowels in the suffix depends largely on the suffix that you use.

Some suffixes are the -a/-e type.

For example when you make a noun plural, you can only add - ler or -lar, there´s no option with -i/-ý/ü/u.

melek - melekler (melek-ler)

Other suffixes are the the i/ý/ü/u type.

For example when you add -li to make an adjective, you can add -li, -lü, -lý, or -lu - depending on the vowel harmony.

süt - sütlü (süt-lü

So look at the suffix, it will tell you what vowel harmony to use.

Here´s an explanation on the use of buffer letters.

As far as when to drop a vowel or use a buffer letter, you need to learn it while you learning suffixes.

For example, the suffix for "my" and "I am" is -im/ým/um/üm.

If you have a word that ends in a vowel, you drop the vowel, for "my":

araba - arabam (araba-m) - my car

and you add the buffer letter for I am:

anne - anneyim (anne-y-im) - I am a mother

thankyou, i think i understand better now, so do i need to learn each buffer consonant for each suffix individually?  also i still dont know wether to drop a vowel add the buffer consonant, does this also depend on the sufix used? so i will need to learn these individually too? . 

You don´t need to learn the buffer for each suffix, there are general rules to apply, and you can see them under this link:

http://www.turkishclass.com/forumTitle_37115

It will come to you as you´re learning, I wouldn´t even try to memorize it at this point.

And yes, it does depend on the suffix whether you drop a vowel or add a buffer, but that´s something you will learn as you go as well.

It´s not something you need to stress about right now. After a few weeks you will know which suffix to use, I promise. It sounds intimidating when you learn about the vowel harmony, but it really isn´t, you will instinctively know which letter to use. And if you make mistakes (as we all do), you´re still going to be understood, so no worries.

Also, if you have specific examples of where you run into a problem you´d like help with, that would help to explain. Sometimes it´s easier to understand general rules on examples.